The Earth’s biggest magmatic events are thought to originate from massive melting when mantle plumes reach the base of the lithosphere. These hot, buoyant upwellings can recycle chemical heterogeneities from the deep lower mantle and carry them toward the surface, providing a window to the composition of the lowermost mantle. Here, we investigate plume dynamics from the core-mantle boundary to the surface using a combination of geodynamic models and seismic observations. Mantle plumes that best fit observations of low premagmatic surface uplift and geochemical data contain up to 15–20% of recycled oceanic crust in the form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. They have columnar tails with a diameter of more than 500 km that remain stable in the mantle for tens or hundreds of millions of years. Consequently, they should be easier to resolve with seismic methods than classical narrow plume tails. Our models show how large fractions of basaltic material exceeding 40% can accumulate within plumes in the upper mantle, explaining the seismologic detection of the X-discontinuity, and providing insights into how recycled material is carried towards the surface. Our findings provide an important step towards constraining the nature of the heterogeneities within mantle plumes and their influence on the thermal, compositional and dynamic evolution of the Earth.
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